Pigments exhibit certain colors because they selectively reflect and absorb certain wavelengths of light. White light is an approximately equal mixture of the entire visible spectrum of light. When white light encounters a colored pigment, some wavelengths are absorbed as they interact with the electronic structure of the pigment. These interactions are determined by the chemistry and bonding of the pigment. The wavelengths not absorbed are reflected back to the observer, and this reflected visible light spectrum creates the appearance of a color. For example, ultramarine reflects blue light, typically between 400 and 500 nanometers in wavelength, and absorbs visible light of other wavelengths.
The appearance of pigments is also dependant upon the spectrum of the source light. Sunlight has a high color temperature, and a fairly uniform spectrum, and is considered a standard for white light. Artificial light sources, including fluorescent light, tend to have great peaks in some regions of their spectrum, and deep valleys in other regions. Viewed under these conditions, pigments may exhibit different colors.
Color spaces used to represent colors numerically, must specify their light source. Lab color measurements, unless otherwise noted, assume that the measurement is taken under a D65 light source, or “Daylight 6500 K”, which is approximately the color temperature of sunlight.
Other properties of a color, such as its saturation or lightness, may be tailored by other substances that typically accompany pigments. Binders and fillers added to pure pigment chemicals also have their own reflection and absorption patterns, which can affect the final spectrum. Likewise, in pigment/binder mixtures, individual rays of light may not encounter pigment molecules, and may be reflected as is. These stray rays of source light contribute to the saturation of the resulting color. Pure pigment allows very little white light to escape, producing a highly saturated color. A small quantity of pigment mixed with a relatively large proportion of white binder, however, will appear desaturated and pale, due to the high quantity of escaping white light.
Pigment particles may be coated or otherwise combined with one or more additives to selectively alter the properties of the pigment. A pigment that has received much attention in this regard is titanium dioxide, TiO2. This pigment is noteworthy for its wide range of applications, from paint to sunscreen to food coloring. Numerous additives and treatments have been described for modifying the properties, and typically the optical characteristics, of pigments such as titanium dioxide. Many of these treatments involve the use of phosphorous or phosphate anions. U.S. Pat. No. 2,817,595 describes a method for preparing an improved titanium dioxide pigment by calcining hydrous titanium oxide in the presence of certain phosphorous compounds. U.S. Pat. No. 3,330,798 describes depositing aluminum phosphate on titanium dioxide pigment particles. U.S. Pat. No. 3,926,660 describes precipitating a metal phosphate on a titanium dioxide pigment. U.S. Pat. No. 3,946,134 is directed to forming a protective coating of a certain phosphate complex on pigment particles. U.S. Pat. No. 4,052,224 describes treating titanium dioxide pigment with a phosphorous compound. U.S. Pat. No. 4,239,548 describes titanium dioxide pigment having one or more coatings containing phosphate radicals. U.S. Pat. No. 4,461,810 describes titanium dioxide particles coated with various anions including phosphate ions. U.S. Pat. No. 5,114,486 describes treating metal oxide pigments such as titanium dioxide with zinc phosphate. U.S. Pat. No. 5,942,281 describes treatment of titanium dioxide pigment by coating with alumina phosphate. U.S. Pat. No. 6,342,099 is directed to coated titanium dioxide pigments from multiple deposits of phosphate compounds.
Efforts have also been directed to altering the physical properties of other pigments besides titanium dioxide. WO 98/38256 describes various inorganic pigments coated with certain oxides or phosphates. U.S. Pat. No. 6,261,691 describes powder coated particles containing a layer of a metal phosphate.
Iron oxide pigments are widely used in ceramic applications, particularly in glazing. Many metal oxides provide the colors in glazes after being fired at high temperatures. Natural iron oxide pigments are called ochres. Many classic paint colors, such as raw and burnt siennas and umbers, are iron oxide pigments. Iron (III) oxide is typically used. Iron pigments are also widely used in the cosmetic field. They are considered to be nontoxic, moisture resistant, and nonbleeding. Typically, the iron (II) oxide pigment is black, while the iron (III) oxide is red or rust colored. Chromium oxide pigments are also widely used in paints, inks, and glasses. Chromium oxides often impart a green color as in CICP Green 17.
A few isolated disclosures are known in which iron or chrome based pigment particles are treated to alter their properties. For example, U.S. Pat. No. 2,419,094 describes the addition of phosphate agents to chromium-containing pigments, including chromite (Cr2O3.FeO). Although primarily directed to treating titanium dioxide pigment, U.S. Pat. No. 3,767,455 notes that chromic oxide or iron oxide pigment may be coated with a phosphate.
RU 2236391 describes grains for incorporating in building materials such as concrete products and roofing materials for increased “coloration intensity.” The grains include a coating of a phosphate binder and pigment such as iron oxide and chromium oxide. U.S. Pat. No. 7,060,126 describes multilayered luster pigments which have beneficial optical properties. The “luster pigments” are metal particles having an outer “(B)” layer which can include various iron oxides such as those of iron and chromium. An outermost “(C)” layer may also include Fe2O3 or Cr2O3 which “may be phosphate-containing.”
Although satisfactory in many regards, as far as is known, the prior art does not provide specific treatment strategies for improving the optical properties of iron or chrome based pigments, and particularly, for improving the solar reflectance of such pigments. Thus, it would be beneficial to provide such improvements for iron and chrome based pigments, so that superior products and applications using such pigments can be realized.